Many studies have been carried out to develop a semi-conductor light emitting device having an LED chip with its light emitting portion made of nitride semi-conductor material such as GaN, InGaN, AlGaInN, for achieving its high light extraction efficiency and high light output. Many studies have been made to develop a light emitting apparatus which radiates mixed-color light by mixture of luminescent light of the semi-conductor light emitting device and light of phosphor as wavelength transforming material which emits light having its wavelength longer than that of luminescent light of the semi-conductor light emitting device. For example, a white colored light emitting device (generally referred to as a white LED) is commercially available for achieving white colored light by mixture of luminescent lights of phosphor and blue or purple colored light of semi-conductor light emitting device.
FIG. 3 shows an example of semi-conductor light emitting device including a sapphire substrate 1′, a cathode 4′, an anode 5′, and a light emission layer 2′ formed of a laminate composed of a buffer layer 121, an n-type GaN layer 122, a light emitting layer 123, and a p-type GaN layer 124. The cathode is provided on the n-type GaN layer 122 at its one surface away from the sapphire substrate 1′. The anode 5′ is formed on the p-type GaN 124. Various laminate structures of the light emission layer 2′ have been proposed.
The semi-conductor light emitting device in FIG. 3 is configured to emit a light by recombination of electron-hole pairs in the light emitting layer 123 in the presence of forward biased voltage applied between the anode 5′ and the cathode 4′. In this semi-conductor light emitting device in FIG. 3, one surface of the sapphire substrate 1′ serves as a light output surface by flip-flip mounting to radiate therethrough a light which is emitted from the light emitting layer 123.
The semi-conductor light emitting device in FIG. 3 has the anode 5′ and the cathode 4′ which are respectively formed of laminates having different electrode structures, due to different electrical characteristic between the p-type GaN 124 and n-type GaN 122 layers. Namely, the electrode suitable for forming the p-type GaN layer 124 is not good ohmic contact (not ohmic contact with low contact resistance) with the n-type GaN layer 122, and the electrode suitable for forming the n-type GaN layer 122 is not good ohmic contact (not ohmic contact with low contact resistance) with the p-type GaN layer 124. In the semi-conductor light emitting device in FIG. 3, the anode 5′ is formed of laminate of Ni film 151, Au film 152, and Al film 153, while the cathode 4′ is formed of Ti film 141 and Al film 142 superimposed on the Ti film 141. Various electrode structures have been proposed, such as Ni/Au, Pd/Ag, Pt/Au film laminates for both of the anode 5′ and the cathode 4′, Ti/Al/Ni/Au film laminate for the cathode 4′.
In producing the semi-conductor light emitting device in FIG. 3, the cathode 4′ and the anode 5′ need to be individually prepared by use of electronic beam deposition method, increasing consumption of metallic material, such as base metal (e.g., Al), novel metal (e.g., Au, Ag, and Pt), and rare metal (e.g., Ti, Ni, and Pd) as well as steps of producing, thereby rising the manufacturing cost.
JP Unexamined patent publication 2004-179347 discloses that both cathode and anode are formed of laminates composed of ITO and Ag films.
However, in the semi-conductor light emitting device of JP Unexamined patent publication 2004-179347, Ag film can be detached from ITO film of laminate for lack of adhesion between ITO and Ag films, during or after flip-flop mounting.
Moreover, the semi-conductor light emitting device in JP Unexamined patent publication 2004-179347 fails to achieve high light output, for large amount of light totally reflected at an interface between the LED layer 2′ and the sapphire substrate 1′ due to a large gap in refractive index therebetween.